Chromium stainless steel with a low chromium content and a low nickel content is extremely advantageous cost-wise compared with austenitic stainless steel such as SUS304 steel, so is suitable for applications of use in large quantities such as structural steels. Such low chromium stainless steel has a ferritic structure or martensitic structure corresponding to the composition of ingredients. In general, ferritic or martensitic stainless steel is inferior in low temperature toughness or corrosion resistance of weld zones. For example, in the case of martensitic stainless steel such as SUS410, the C content is a high one of 0.1 mass % or so, so the steel is inferior in weld zone toughness or weld zone workability and, in addition, preheating is required at the time of welding and the welding work efficiency is inferior as well, so problems remained in application to materials requiring welding.
As a means for preventing such deterioration of characteristics of the weld zones, the method, such as described in Japanese Patent Publication (B2) No. 51-13463 and Japanese Patent Publication (B2) No. 61-23259, of using the martensitic structure formed at the weld zones to prevent a drop in the corrosion resistance and low temperature toughness has been disclosed. Japanese Patent Publication (B2) No. 51-13463 proposes the method of including Cr: 10 to 18%, Ni: 0.1 to 3.4%, Si: 1.0% or less, and Mn: 4.0% or less and further reducing C to 0.030% or less and N to 0.020% or less in the steel ingredients and forming a massive martensitic structure at the weld heat affected zones. Due to this, martensitic stainless steel for welded structures improved in performance of the weld zones is provided.
Such low chromium stainless steel using martensitic transformation in the weld zones is actually being used as beams for marine containers. Up until now, there has never been any example where the corrosion resistance or low temperature toughness at the weld zones became a problem. However, in the case of use under a harsh corrosive environment (where the steel material is wet for a long time, the chloride concentration is high, a high temperature, a low pH, etc.), it has been revealed that the corrosion resistance at the weld zones is insufficient. For example, in the case of use at the beds of railroad cars carrying coal or iron ore, it has been reported that grain boundary corrosion occurs at the weld heat affected zones.
As the method of improving the corrosion resistance of weld heat affected zones or the weld zone toughness of low chromium stainless steel, the above-mentioned higher purity and, further, in addition to this the addition of elements for fixing carbon or nitrogen as carbides or nitrides are effective, so various steels produced by this means have been disclosed. For example, Japanese Patent Publication (A) No. 2002-327251 discloses the addition of suitable quantities of the carbon and nitrogen stabilizing elements Nb and Ti so as to prevent the deterioration of the grain boundary corrosion resistance of the weld zones of low chromium stainless steel using martensitic transformation and thereby obtaining low chromium stainless steel superior in low temperature toughness. Japanese Patent No. 3491625 similarly discloses an Fe—Cr alloy obtained by adding the carbonitride-forming elements Ti, Nb, Ta, and Zr and thereby improved in weld zone corrosion resistance. However, this patent requires the inclusion of Co, V, and W and has as its object the improvement of the resistance to initial rust formation.
With the above as background, in recent years, in environments of use for railroad car beds for coal or iron ore mined inland and transported by rail to the shore etc., as a measure against grain boundary corrosion of the weld heat affected zones, there is the example of use of low chromium stainless steel to which Ti is added in the same way as the disclosures of Japanese Patent Publication (A) No. 2002-327251 and Japanese Patent No. 3491625.
However, in this example, the weld heat affected zones are improved in grain boundary corrosion resistance, but the inventors newly discovered that there is a problem with occurrence of preferential corrosion at the weld zones and the heat affected zones most adjoining them, that is, near the locations along the interface with the massive martensitic structure (fusion lines). This phenomenon, as disclosed in the Journal of the JWS, vol. 44, 1975, no. 8, p. 679, is similar to the phenomenon called “knife line attack” seen in weld zones of SUS321 or SUS347 stable austenitic stainless steel. Corrosion proceeds preferentially at the interfaces (fusion lines) between the weld zones and heat affected zones and the corroded regions expand, so this is a problem which should be improved on.
Knife line attack is caused during the welding of stainless steel fixing C by TiC or NbC by the TiC or NbC becoming solid solute in the region where the heat history is raised to about 1200° C. or more and then the Cr carbides precipitating at the crystal grain boundaries and the corrosion resistance dropping when passing through the sensitization temperature region in the subsequent cooling process. However, in the case of low chromium stainless steel, for what sort of reasons preferential corrosion occurs has not been sufficiently studied. Countermeasures have not been devised either.
Further, the above-mentioned low chromium stainless steel to which C- and N-fixing elements are added is of a system of ingredients improving the grain boundary corrosion resistance of the weld zones, but the corrosion resistance of the heat affected zones after several welding operations can hardly be said to be sufficient. It has been reported that corrosion sometimes occurs at the weld heat affected zones. From the viewpoints of increase the freedom of design of welded structures and of improving the ease of weld repair, a low chromium stainless steel enabling multipass welding which is superior in corrosion resistance of the heat affected zones even after multipass welding is being awaited.
On the other hand, in the production of low chromium stainless steel, it is known that edge cracking easily occurs at the time of hot rolling. This is believed due to the stability of the austenitic phase and the phase of δ-ferrite in the hot working temperature region being directly affected by the change in balance of the contained elements. Accordingly, there are problems to be solved from the viewpoint of the optimization of the production process as well. Improvement has been desired.
Further, in the case of use for the beds of railroad cars carrying coal or iron ore, increasing the load capacity so as to improve the transport efficiency and reducing the weight so as to reduce the fuel consumption etc. have been earnestly desired. The gross weight of railroad cars is fixed, so to raise the load capacity, it is essential to make the stainless steel plate thinner. To realize this, increased strength of low chromium stainless steel plate is essential, but low chromium stainless steel plate superior in strength-ductility balance considering workability as well has not yet been developed. Its appearance has been awaited.